frost free outdoor faucet

If its a 300 and 500 series Mansfield (the most common type here in Chicago) it sounds like you may have a broken vacuum breaker (I often see this at home inspections), if so there is a repair kit:

Home Inspection: "A business with illogically high liability, slim profit margins and limited economies of scale. An incredibly diverse, multi-disciplined consulting service, delivered under difficult in-field circumstances, before a hostile audience in an impossibly short time frame, requiring the production of an extraordinarily detailed technical report, almost instantly, without benefit of research facilities or resources." - Alan Carson